Road surface unevenness, or road roughness, has a direct impact on the economics and efficiency of a nation's road transport system. It is an important indicator of road condition, in terms of road pavement performance, and as a major determinant of road user costs—fuel consumption and gaseous emissions, and vehicle wear and tear—and the relative degree of comfort or discomfort and fatigue experienced by people travelling.
To gauge the condition of the road network, government road agencies commission regular network level road roughness surveys to be conducted, commonly annually or biennially, and use the information from these surveys to allocate substantial funds to both maintain and further develop the road asset. The roughness surveys typically may cover in excess of 20,000 lane-kilometers and they are performed with highly sophisticated road profile measuring equipment travelling at highway speed. The cost of such surveys is typically about AU$800,000 or 20,000 lane kilometers and such surveys are expected to produce accurate and reliable road roughness information.
Validation of road roughness measurements carried out before, during and at the conclusion of major surveys continues to be hampered by the lack of suitable equipment for taking reference longitudinal profiles, particularly on highly textured surfaces. This can lead to anomalous roughness readings and discontinuities in historical roughness data. When problems occur they are generally not easily identified and corrected, and serious contractual issues can consequently arise between the government road agencies and the service provider.
Limitations of current reference profilers used for validating road roughness measuring equipment can be summarized as follows:                The horizontal distance increment at which height measurements are taken is too coarse, typically being greater than about 240 mm. On some surfaces this causes localised roughness features to be missed or poorly reproduced. These same features are recorded by highway-speed laser-based profilers, which are used almost exclusively for network level road roughness surveys worldwide, and whose measurements should be validated, using a reference profiler, before during and at the conclusion of major surveys.        The sensitivity of current reference profilers to coarsely textured surfaces is different to that of highway-speed laser-based profiling systems. This difference in sensitivity leads to equipment validation problems.        Current reference profilers can produce a bias error on textured surfaces.        Current reference profilers are sensitive to roughness in the 0.5 m to 50 m horizontal wavelength range only, and are largely insensitive to megatexture, which covers wavelengths in the range 50 mm to 500 mm. Megatexture is becoming of increasing importance to government road agencies.        
An aim of the present invention is to provide a measurement system, particularly using a reference profiler, which overcomes, or at least minimises, these limitations.
Those skilled in the art of measuring pavement surface profiles understand the term “microtexture” to refer to irregularities having a wavelength up to 0.5 mm, the term “macrotexture” to refer to irregularities having a wavelength from 0.5 to 50 mm, and the term “megatexture” to refer to irregularities having a wavelength from 50 to 500 mm. The description in this patent specification follows that terminology. While the word “roughness” is a term commonly used by those skilled in the art to refer to irregularities having wavelength above 500 mm, in this specification the word “roughness” is also used in its general sense as meaning unevenness or the existence of irregularities of any size, unless the aforementioned size-limited meaning is obvious from the context.